Method for smoothing a current consumed by an integrated circuit and corresponding device

ABSTRACT

A method for smoothing current consumed by an electronic device is based on a series of current copying operations and on a current source delivering a reference current. The reference current is delivered in such a manner that current consumed as seen from the power supply depends on the reference current.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 15/150,713 filed on May 10, 2016, which claims the benefit of FR 1559354, filed on Oct. 1, 2015, which applications are hereby incorporated herein by reference.

TECHNICAL FIELD

Various embodiments and their implementation relate to integrated circuits, and more particularly, to integrated circuits with secure modules, and notably the protection of these modules against external attacks of the SPA (Simple Power Analysis) type.

BACKGROUND

An integrated circuit comprising a module can be the target of attacks aimed at recovering secure information. In particular, the attacks may be by analysis of power consumption or SPA.

In operation, an integrated circuit consumes more or less power depending on the operations that it is carrying out. SPA attack comprises the analysis of these variations in power consumption to determine the operations being carried out and/or their occurrences.

In secured applications, it is therefore recommended to smooth the power consumption seen from the power supply. This is done in such a manner to make it more difficult for a potential attacker to determine the activity of the various components of the circuit by SPA attacks.

There is circuitry to protect against SPA attacks, such as shunt regulators, for example. However, circuitry with shunt regulators is not suitable for smoothing the current consumed by several modules.

SUMMARY

According to one embodiment, a different and straightforward approach is provided for smoothing current seen from the power supply of an integrated circuit. This approach is compatible with an integrated circuit comprising several secure modules.

A method for smoothing current consumed by a power supply for an electronic circuit is provided. The electronic circuit may comprise at least one module, such as a microprocessor, memory, etc., powered directly or indirectly by the power supply, via a voltage regulator. The at least one module consumes a module current.

For each module, a module auxiliary current equal to a first fraction of the corresponding module current may be generated within the electronic circuit. The electronic circuit may be equipped with a first stage, powered by the power supply, comprising at least one main current source supplying a main current higher than the sum of the maximum values of each module auxiliary current. The first stage may deliver a stage current at least equal to the main current. An intermediate current equal to a difference between the stage current and a secondary current equal to the sum of each module auxiliary current may be generated. In a terminal stage powered by the power supply, the intermediate current may be multiplied by a multiplication factor equal to an inverse of the first fraction augmented by one.

Thus, in the above method, a plurality of currents consumed by the power supply may be generated, whose sum does not depend on the current consumed by each module but instead on the current supplied by the current source stage. The power consumption of the integrated circuit may therefore be smoothed. This overall smoothed power consumption as externally viewed may be higher than the sum of the maximum power consumptions of each module.

The main current delivered by the main current source may be a second fraction of a reference current. This second fraction may be equal to the first fraction, and of course in this case, the reference current may be higher than the sum of the maximum values of each module auxiliary current.

The first stage may further comprise at least one additional activatable current source delivering an additional current. The stage current may be equal to the sum of the main current and of each additional current delivered by each activated additional current source.

Thus, by activating successively and/or simultaneously the various current sources, the current consumed by the power supply varies. This may render the detection of the variations of the module current even more difficult. In this regard, it is also possible to add capacitive noise within the device, for example, at the output of the first stage.

According to another aspect, an electronic device comprises a power supply terminal, and at least one module connected to the power supply terminal and configured for consuming a module current. First generation means or circuitry may be connected to the power supply terminal and may be configured for generating, for each module, a module auxiliary current equal to a first fraction of the corresponding module current. A first stage, connected to the power supply terminal, may comprise at least a main current source configured for supplying a main current higher than the sum of the maximum values of each module auxiliary current. The first stage may be configured for delivering a stage current at least equal to the main current. Second generation means or circuitry may be configured for generating an intermediate current equal to the difference between the stage current and a secondary current equal to the sum of each module auxiliary current. A terminal stage may be connected to the power supply terminal and may be configured for multiplying the intermediate current by a multiplication factor equal to the inverse of the first fraction incremented by one.

The main current may be equal to a second fraction of a reference current. The main current source may comprise a control input designed to receive a control signal allowing the value of the second fraction to be selected from a set of values. The first fraction may be equal to the second fraction, and in this case, the reference current may be higher than the sum of the module auxiliary currents.

The device advantageously comprises a succession of current copying means or circuitry. Thus, the first generation circuitry may comprise, for each module, a first copy factor current mirror equal to the first fraction. The first copy factor current mirror has a first output delivering the corresponding module current, and a second output delivering the corresponding module auxiliary current.

The second generation circuitry may comprise a second copy factor current mirror equal to one. The second output of the first current mirror may be connected to the input of the second current mirror. The output of the second current mirror may be connected to the output of the first current source stage.

The terminal stage may comprise current copying means or circuitry having a copy factor equal to the multiplication factor. The current copying circuitry may have an input connected to the output of the second generation circuitry, and an output connected to the power supply terminal.

According to one embodiment, the terminal stage may comprise a first MOS transistor connected to the output of the second current mirror, and at least a second MOS transistor connected between the power supply terminal and ground. The gates of the MOS transistors may be connected together.

In addition, to ensure better current copying by the second generation circuitry, notably when the module auxiliary current or the sum of the module auxiliary currents is close to the stage current, an intermediate operational amplifier may also be added. The intermediate operational amplifier may have a non-inverting input connected to the output of the second current mirror, an inverting input connected to the input of the second current mirror, and an output connected to the gates of the MOS transistors of the terminal stage.

Also, to improve the current copying, the first generation circuitry may comprise, for each module, a first cascode stage connected between the second output of the corresponding first current mirror and the input of the second current mirror. The first cascode stage may comprise a first PMOS transistor and a first operational amplifier. The first operational amplifier may have a non-inverting input connected between the power supply terminal and the corresponding module, an inverting input connected to the source of the first PMOS transistor, and an output connected to the gate of the first PMOS transistor.

A second cascode stage may also be connected between the output of the terminal stage and the power supply terminal. The cascode stage may comprise a second PMOS transistor and a second operational amplifier. The second operational amplifier may have a non-inverting input connected between the output of the terminal stage and the second PMOS transistor, an inverting input connected to the input of the terminal stage, and an output connected to the gate of the second PMOS transistor.

The first stage may comprise at least one additional current source activatable independently of the main current source. The outputs of all the current sources may be connected to the output of the first stage.

The electronic device may further comprise a generator of capacitive noise, for example. The capacitive noise generator may be connected to the output of the first stage.

A regulator configured for delivering a regulated voltage to the at least one module may be connected between the power supply terminal and the at least one module. The regulator may comprise the first generation circuitry.

According to one embodiment, the device may comprise a plurality of modules, and the first generation circuitry may comprise a plurality of first outputs respectively connected to the modules to deliver the respective module currents and a plurality of second outputs connected together to the input of the first generation circuitry to deliver the secondary current.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features will become apparent upon examining the detailed description of one non-limiting embodiment and its implementation, and from the appended drawings in which:

FIGS. 1 to 6 show schematically block diagrams of various embodiments according to the present disclosure.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In the following description, the terms coupled and connected denote an electrical link which is either direct or indirect via other devices or means of connection. In FIG. 1, the reference DIS denotes an electronic device according to one embodiment.

The device DIS comprises a module 1, for example, a microprocessor of a smartcard. The device may be situated within the integrated circuit of the smartcard.

The device DIS furthermore comprises a power supply terminal 2 designed to receive a power supply voltage V_(cc), for example, a voltage of 5 Volts. The device DIS also comprises first generation circuitry 4 comprising a first input E31 and a second input E32 connected to the power supply terminal 2. The first generation circuitry 4 also comprises a first output S31 and a second output S32.

The device comprises second generation circuitry 5 comprising an input E5 and an output S5. The device comprises a terminal stage 7 comprising an input E7 and an output S7. The device comprises a first current source stage 6 comprising an input E6 and an output S6. Their respective connections and characteristics will be described in more detail below.

A regulator 3 is connected between the power supply terminal 2 and the microprocessor 1. The connection is in such a manner so as to deliver a regulated voltage V_(dd) to the microprocessor. For example, a voltage of 2.5 Volts may be delivered. The microprocessor 1 consumes a current I_(vdd) whose value depends on the operations that it carries out.

In this example, the regulator 3 comprises the first generation circuitry 4. The first generation circuitry 4 comprises a first current mirror which delivers, at the input E5 of the second generation circuitry 5, an auxiliary current I_(aux) equal to a first fraction of the current consumed I_(vdd). In this example, the current mirror has a copy factor equal to 1/100. In other words, the current mirror delivers a module auxiliary current I_(aux) equal to one hundredth of the current consumed I_(vdd) (the first fraction is equal to 1/100).

The first current mirror conventionally comprises two transistors 41 and 42 coupled together via their gates. The first transistor 41, for example a PMOS transistor, is configured as a diode. The first transistor 41 has a source S₄₁, which forms the first input E31 of the first generation circuitry, connected to the first power supply terminal, and a drain D₄₁, which forms the first output S31 of the first generation circuitry, connected to the module 1.

The second transistor 42, for example a second PMOS transistor, has a source S₄₂, which forms the second input E32 of the first generation circuitry 4, connected to the power supply terminal 2, and a drain D₄₂, which forms the second output S32 of the first generation circuitry, connected to the input E5 of the second generation circuitry 5.

To obtain the first fraction of the consumed current I_(vdd), the second PMOS transistor may be chosen having a ratio W₄₂/L₄₂ between the width W₄₂ and the length L₄₂ of its channel 100 times smaller than the ratio W₄₁/L₄₁ between the width W₄₁ and the length L₄₁ of the channel of the first transistor 41.

An alternative approach would be to have a plurality of first transistors 41 connected in parallel and one or more second transistors 42 identical to the first transistors 41 connected in parallel. The connection is in such a manner that the number of first transistors is 100 times greater than the number of second transistors.

In this example, the second generation circuitry 5 comprises a second current mirror, with a copy factor equal to one. The second current mirror conventionally comprises two identical transistors 51 and 52, for example NMOS transistors, coupled together by their gates G₅₁ and G₅₂.

The first NMOS transistor 51 is configured as a diode. The first NMOS transistor 51 has a drain D₅₁, which forms the input E5 of the second generation circuitry 5, connected to the drain D₄₂ of the second PMOS transistor 42 of the first current mirror, and a source S₅₁ connected to ground GND.

The second NMOS transistor 52 has a source S₅₂ connected to ground, and a drain D₅₂, which forms the output S5 of the second generation circuitry 5, connected to the input E7 of the terminal stage 7 and to the output S6 of the first stage 6. Thus, the second generation circuitry receives the module auxiliary current I_(aux) at the input E5 and copies this same module auxiliary current I_(aux) at the output S5.

The first stage 6 comprises a main current source 61 connected between the power supply terminal 2 and the input E7 of the terminal stage. The main current source 61 is configured for delivering a main current I_(p), equal to a second fraction of a reference current I_(set), to the input E7 of the terminal stage 5.

The first fraction is chosen equal to the second fraction. Thus, in this example where the module auxiliary current I_(aux) is equal to

$\frac{Ivdd}{100},$ the main current I_(p) is equal to

$\frac{Iset}{100}.$ The reference current I_(set) is chosen to be higher than the maximum value of the module current I_(vdd).

The maximum value of the module current I_(vdd) is, for example, determined by simulation during the design of the integrated circuit taking into account the foreseeable activity of the module 1. Since the reference current I_(set) is higher than the maximum value of the maximum module current, the main current I_(p) is higher than the maximum value of the module auxiliary current I_(aux).

The main current source 61 may further comprise a control input 62 designed to receive a control signal SC allowing the second fraction to be chosen from among a predefined set of values. For example, the set of values can be { 1/100; 1/60; 1/50; 1/40}, and the main current I_(p) delivered can therefore be equal to

$\frac{Iset}{100},{{or}\mspace{14mu}\frac{Iset}{60}},$ etc.

Since the first stage 6 delivers a main current I_(p) equal to

$\frac{Iset}{100},$ the terminal stage 7 therefore receives a positive intermediate current I_(int) equal to

$\frac{Iset}{100} - \frac{Ivdd}{100}$ at its input E7. The terminal stage 7 comprises, in this example, a third copy factor current mirror equal to the inverse of the first fraction incremented by 1. The third current mirror therefore has a copy factor equal to 101.

The third current mirror conventionally comprises a third and a fourth transistor 71 and 72, for example NMOS transistors, coupled together via their gates G₇₁ and G₇₂.

The third NMOS transistor 71 is configured as a diode. The third NMOS transistor 71 has a source S₇₁ connected to ground, and a drain D₇₁, which forms the input E7 of the terminal stage, connected to the output S5 of the second generation circuitry and to the output S6 of the first stage. The fourth NMOS transistor 72 has a source S₇₂ connected to ground GND and a drain D₇₂ connected to the power supply terminal 2.

To obtain a multiplication factor equal to 101, a fourth NMOS transistor 72 can be chosen whose ratio W₇₂/L₇₂ between the width W₇₂ and the length L₇₂ of its channel is 101 times greater than the ratio W₇₁/L₇₁ between the width W₇₁ and the length L₇₁ of the channel of the third transistor 71.

An alternative approach would be to have one or more third transistors 71 connected in parallel and a plurality of fourth transistors 72, identical to the third transistors 71, connected in parallel in such a manner that the number of fourth transistors is 101 times higher than the number of third transistors.

Thus, the terminal stage generates a current I_(term) equal to 101 times the intermediate current I_(int). The current

$I_{term} = {101*{\frac{{Iset} - {Ivdd}}{100}.}}$

In operation, the device DIS thus configured therefore consumes the first current I_(vdd), the module auxiliary current

${I_{aux} = \frac{Ivdd}{100}},$ the main current

${I_{p} = \frac{Iset}{100}},$ and the terminal current I_(term)=101*(I_(p)−I_(aux)).

The current I_(vcc) consumed by the power supply is therefore equal to the sum of these currents, i.e., 1.02*I_(set), and does not depend on the module current I_(vdd) but only on the reference current I_(set). The reference current I_(set) is constant and higher than the maximum value of the module current V_(vdd).

FIG. 2 illustrates a second embodiment. In this embodiment, the device DIS as previously described has been modified in such a manner that the third NMOS transistor 71 of the terminal stage is no longer configured as a diode. An operational amplifier 8 has been added between the second generation circuitry 5 and the terminal stage 7.

The operational amplifier 8 has a non-inverting input connected to the input E7 of the terminal stage, and an inverting input connected to the mutually-coupled G₅₁ and G₅₂ gates of the first and second NMOS transistors 51 and 52. In other words, connection is to the first input E5 of the second generation circuitry 5 since the first NMOS transistor 51 is configured as a diode. The output of the amplifier 8 is connected to the mutually-coupled gates G₇₁ and G₇₂ of the third NMOS transistor 71 and of the fourth NMOS transistor 72.

Thus, by driving the gate G₇₁ of the third NMOS transistor 71 in such a manner as to equalize the potentials of the drains D₅₁ and D₅₂ of the first and second NMOS transistors 51 and 52, the operational amplifier allows an intermediate current I_(int) to be obtained. The intermediate current I_(int) is obtained at the input E7 of the terminal stage which is precisely equal to the difference between the main current I_(p) and the module auxiliary current I_(aux). This is true even if the module auxiliary current I_(aux) has a value close to the main current I_(p).

According to another embodiment illustrated in FIG. 3, the device DIS may also comprise a first cascode stage 9 coupled to the second output S32 of the first generation circuitry 4, and a second cascode stage 10 coupled to the output S7 of the terminal stage.

The first cascode stage 9 comprises a third PMOS transistor 91 having a source S₉₁ connected to the second output S32 of the first generation circuitry, and a drain D₉₁ coupled to the input E5 of the second generation circuitry 5. The cascode stage 9 further comprises a second operational amplifier 92 having a non-inverting input coupled to the first output S31 of the first generation circuitry 4, and a non-inverting input coupled to the second output S32 of the first generation circuitry 4. The output of the second operational amplifier 92 is coupled to the gate G₉₁ of the third PMOS transistor 91.

Thus, the first cascode stage 9 allows the voltages at the first and second outputs S31 and S32 of the first generation circuitry to be equalized, which contributes to obtaining the desired ratio of 1/100 between the current I_(vdd) and the auxiliary current

$I_{aux} = {\frac{I_{vdd}}{100}.}$

The second cascode stage 10 comprises a fourth PMOS transistor 101 having a source S₁₀₁ connected to the first power supply terminal 2, and a drain D₁₀₁ connected to the output S7 of the terminal stage.

The second cascode stage 10 further comprises a third operational amplifier 102 having a non-inverting input connected to the output S7 of the terminal stage, and an inverting input connected to the input E7 of the terminal stage. The output of the amplifier is connected to the gate G₁₀₁ of the fourth PMOS transistor 101.

Thus, the second cascode stage 10 allows the voltages at the input E7 and at the output S7 of the terminal stage to be equalized. This contributes to obtaining the desired ratio of 101 between the intermediate current I_(int) and the terminal current I_(term).

The two cascode stages 9 and 10 therefore allow the precisions of the current copying operations respectively carried out by the first generation circuitry 4 and of the terminal stage 7 to be improved.

FIG. 4 illustrates one embodiment in which the first stage 6 has been modified with respect to the device DIS previously described and illustrated by FIG. 1. In this embodiment, the first stage comprises, in addition to the main current source 61 delivering the second fraction of the reference current I_(set), a first additional current source 63 delivering a first additional current I₁ and a second additional current source 64 delivering a second additional current I₂.

The three current sources, 61, 63, and 64 are configured in parallel between the power supply terminal 2 and the output S6 of the first stage 6. The first and the second additional sources 63 and 64 are furthermore activatable independently of the main current source 61.

The stage current I_(stage) delivered by the first stage 6 is therefore a current equal to the sum of the main current I_(p), of the first additional current I₁ and of the second additional current I₂ when the two sources of additional currents 63 and 64 are activated. The total current I_(vcc) consumed by the power supply is then I_(vcc)=1.02*Iset+102*I₁+102*I₂.

Since the first and second additional current sources 63 and 64 may be independently activated, the current I_(vcc) consumed by the power supply may successively take various values from among the following set of values: I _(vcc)=1.02*Iset+102*I ₁+102*I ₂, I _(vcc)=1.02*Iset+102*I ₁, I _(vcc)=1.02*Iset+102*I ₂, I _(vcc)=1.02*Iset.

Thus, the detection of the variations of the module current I_(vdd) are even more difficult to detect by attacks based on analysis of power consumption (SPA). This modification of the first stage is compatible with the embodiments illustrated in FIGS. 2 and 3. It is presented here using the embodiment illustrated in FIG. 1 for purposes of simplification.

According to one variation illustrated in FIG. 5, it is also possible to add a generator of capacitive noise at the output S6 of the first stage. For example, the generator of capacitive noise 11 comprises a capacitor 110, an inverter 111 and control means or controller 112.

The first capacitor is connected via a first terminal to the output S6 of the first stage and via a second terminal to the inverter 111. The inverter 111 is powered between the power supply terminal 2 and ground GND. The controller 112 is connected to the inverter 111 so as to, depending on the output of the inverter, charge or discharge the capacitor 110. This embodiment is also compatible with the embodiments in FIGS. 1-4.

According to one variation, the device can comprise several modules. For example, in the embodiment illustrated by FIG. 6, the device DIS comprises a second module 12, for example a second microprocessor, which consumes a second module current I_(vdd2), and a second regulator 13 coupled between the power supply terminal 2 and the second module, also comprising the first generation circuitry 14.

The first generation circuitry 14 therefore delivers a second module auxiliary current I_(aux2) equal to the first fraction of the second module current I_(vdd2) at the input E5 of the second generation circuitry 5. In this example,

$I_{{aux}2} = {\frac{I_{{vdd}2}}{100}.}$

Thus, the second generation circuitry 5 receives a secondary current I_(see) on their input E5 equal to the sum of the auxiliary currents of module I_(aux) and I_(aux2). The main current I_(p) is, in this example, slightly higher than the maximum value of the secondary current I_(sec).

This embodiment is compatible with all of the embodiments described previously and illustrated by FIGS. 1-5. Although embodiments have been described in which the current regulators 3 and 13 comprise the first generation circuitry 4 and 14, an embodiment in which the regulators 3 and 13 are separate from the first generation circuitry 4 and 14 may be implemented.

Furthermore, the regulators are not indispensable. Indeed, each module could be powered by the power supply voltage V_(cc) via a current mirror. Furthermore, although an efficient injection of capacitive noise at the output S6 of the first stage 6 has been described, capacitive noise could also be injected at other nodes, for example, at the input E5 of the second generation circuitry 5 or at the output of the operational amplifier 8.

While this invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments. 

What is claimed is:
 1. A method comprising: providing a main current to an electronic device from a power supply; and generating a smoothing current from the power supply, wherein a sum of the main current and the smoothing current is proportional to a reference current irrespective of variations of the main current, and wherein generating the smoothing current comprises: generating a first current proportional to the main current; generating a second current proportional to the reference current; and generating a third current proportional to a difference between the reference current and the main current.
 2. The method of claim 1, wherein the first current is equal to the main current divided by a first factor; and the second current is equal to the reference current divided by the first factor.
 3. The method of claim 2, wherein the first factor is equal to
 100. 4. The method of claim 1, further comprising generating an independent current from the power supply.
 5. A method comprising: providing a main current to an electronic device from a power supply; and generating a smoothing current from the power supply, wherein a sum of the main current and the smoothing current is proportional to a reference current irrespective of variations of the main current, wherein the reference current is higher than a maximum value of the main current.
 6. A circuit comprising: an electronic device configured to consume a main current from a power supply; and a smoothing current generator circuit configured to consume a smoothing current from the power supply, wherein a sum of the main current and the smoothing current is proportional to a reference current irrespective of variations of the main current, wherein the smoothing current generator circuit comprises: a first current generation circuit configured to generate a first current proportional to the main current; a second current generation circuit configured to generate a second current proportional to the reference current; and a third current generation circuit configured to generate a third current proportional to a difference between the reference current and the main current.
 7. The circuit of claim 6, wherein the first current generation circuit comprises a first current mirror; the second current generation circuit comprises a second current mirror; and the third current generation circuit comprises a third current mirror.
 8. The circuit of claim 7, wherein the first current mirror comprises transistors of a p-type; the second current mirror comprises transistors an n-type; and the third current mirror comprises transistors of the n-type.
 9. The circuit of claim 8, wherein the smoothing current generator circuit further comprises a noise generator coupled to the second current mirror.
 10. The circuit of claim 7, wherein the first current mirror is coupled to the second current mirror; and the second current mirror is coupled to the third current mirror.
 11. The circuit of claim 10, wherein the smoothing current generator circuit further comprises a first operation amplifier coupled between the second current mirror and the third current mirror.
 12. The circuit of claim 11, wherein the smoothing current generator circuit further comprises a first cascode stage coupled between the first current mirror and the second current mirror; and a second cascode stage coupled between the power supply and the third current mirror.
 13. The circuit of claim 6, wherein the electronic device comprises a microprocessor.
 14. A circuit comprising: an electronic device configured to consume a main current from a power supply; and a smoothing current generator circuit configured to consume a smoothing current from the power supply, wherein a sum of the main current and the smoothing current is proportional to a reference current irrespective of variations of the main current, wherein the reference current is higher than a maximum value of the main current.
 15. A smartcard comprising: a microprocessor configured to consume a main current from a power supply; and a smoothing current generator circuit configured to consume a smoothing current from the power supply, wherein a sum of the main current and the smoothing current is proportional to a reference current irrespective of variations of the main current, wherein the smoothing current generator circuit comprises: a first current generation circuit configured to generate a first current proportional to the main current; a second current generation circuit configured to generate a second current proportional to the reference current; and a third current generation circuit configured to generate a third current proportional to a difference between the reference current and the main current.
 16. The smartcard of claim 15, wherein the smoothing current generator circuit further comprises a noise generator coupled to the second current generation circuit.
 17. The smartcard of claim 15, wherein the reference current is higher than a maximum value of the main current.
 18. The circuit of claim 14, wherein the smoothing current generator circuit comprises: a first current generation circuit configured to generate a first current proportional to the main current; a second current generation circuit configured to generate a second current proportional to the reference current; and a third current generation circuit configured to generate a third current proportional to a difference between the reference current and the main current.
 19. The circuit of claim 18, wherein the first current generation circuit comprises a first current mirror; the second current generation circuit comprises a second current mirror; and the third current generation circuit comprises a third current mirror.
 20. The circuit of claim 19, wherein the first current mirror is coupled to the second current mirror; the second current mirror is coupled to the third current mirror; and the smoothing current generator circuit further comprises: a first operation amplifier coupled between the second current mirror and the third current mirror, a first cascode stage coupled between the first current mirror and the second current mirror, and a second cascode stage coupled between the power supply and the third current mirror. 